1. Field of the Invention
The invention relates to a process for the production of a material having an improved dielectric strength, the material itself and the use of the material obtained by the aforementioned process in the manufacture of power transmission cables or any other device requiring an electrical insulation.
More specifically, the invention relates to a process making it possible to obtain such a material formed from a conductive polymer (single conductive polymer grafted onto an insulating polymer or copolymer containing one or more combined systems), dispersed in an insulating polymer or in an insulating polymer mixture.
2. Description of the Background
Generally, defects present in the natural state in insulating polymers and materials, such as e.g. impurities or cavities resulting from synthesis or shaping of the material, lead to local charge build-ups when said material is exposed to an electric field. Such charge build-ups can lead to material breakdown phenomena. Therefore materials having improved breakdown resistance characteristics have been developed and can be used in any device requiring an electrical insulation and in particular in very high voltage cables. Such a cable generally comprises a conductive core covered with a semiconductor screen and then an insulating material sheath, finally followed by an external protective sheath. In such cables it is necessary to have low dielectric losses and constants in order to bring about the transmission of power over long distances.
EP 507 676 discloses a material for a semiconductor screen usable in the manufacture of high voltage cables and in the electromagnetic shielding of electrical and electronic equipment. This material has a conductivity below 10.sup.-8 S/m, but which is liable to rise under the effect of an electric field. This material incorporates an insulating polymer matrix into which is incorporated at least 5 to 70 wt. % of a second undoped or dedoped, conjugate polymer, which is not intrinsically conductive and having a conductivity below 10.sup.-4 S.m.sup.-1, but liable to increase under the effect of an electric field.
However, the production of such materials by the direct mixing of powders of different constituents does not make it possible to obtain the desired improvement to the dielectric properties of the material.
WO 89/01015 discloses a composite composition of an insulating polymer and a conductive polymer based on poly(alkylthiophene) doped by an electron donor or acceptor. The production of this material is brought about by mixing insulating polymer and conductive polymer powders. The mixture obtained is shaped by conventional polymer production methods. The aim is to obtain mixtures having conductivity levels generally between 10.sup.-10 and 100 S.cm.sup.-1.
EP 385 523 discloses a process for the preparation of electrically conductive polymers derived from 3-alkylthiophenes consisting of carrying out the chemical polymerization of the 3-alkylthiophene by means of a ferric salt, an alkyl halide and water. The thus obtained poly(alkylthiophenes) can be mixed with insulating polymers of the polyethylene type and hot compressed in order to obtain composite objects or plates. The aim is to obtain mixtures with a conductivity of approximately 10.sup.-6 to 1 S.cm.sup.-1.
However, in general terms, the mixing of conductive polymer powders at levels of approximately 10 to 1000 ppm in insulating polymers normally leads to materials which appear heterogeneous at a scale of approximately 1 micron when observed in scanning electron microscopy (cf. attached FIG. 1). At such low charge levels, a poor dispersion at a scale of 1 micron does not make it possible to obtain the sought improvement to the dielectric properties.